Resistors in the prior art have had the magnitudes of their resistance dependent upon their operating temperature, as characterized by their temperature coefficient of resistance, TCR. Many attempts have been made in the prior art to reduce the temperature coefficient of resistance.
Well known circuit techniques have been used in the prior art to compensate for changes in resistance due to temperature by placing a PN junction with an opposing temperature coefficient of impedance in series with the resistor.
These prior art attempts to solve the problem of resistance variation with temperature are limited in the range of resistance which can be compensated, in the non-linear characteristic of the series impedance, in the temperature range over which the TCR can be compensated and in the requirement for a compensation device for each resistor to be compensated.
Other examples of attempts to compensate for temperature change are covered in several patents. U.S. Pat. No. 3,683,306 to Bulthuis discloses the use of ion implantation to induce damage in the region within which the diffused resistor is formed so as to get a zero temperature coefficient of resistance.
U.S. Pat. No. 3,947,866 to Stellrecht discloses an ion implanted diffused resistor having a central portion which is deeper than a peripheral portion so that the central portion has a negative temperature coefficient of resistance and the peripheral portion has a positive temperature coefficient of resistance. The specification states that a net zero temperature coefficient of resistance can be obtained.
These prior arts attempt to solve the problem of a variation in magnitude of the resistance with operating temperature have raised still other problems such as increased processing complexity, limited range of resistivity, and limited temperature range over which the desired compensation takes place.